Thin-walled structures, combining a variety of thin-walled shapes with the mechanical strength of monoliths, have diverse technological and engineering applications. Typical applications for such materials include gas and liquid flow dividers used in heat exchangers, mufflers, filters, catalytic carriers used in various chemical industries and in emission control for vehicles, etc. In many applications, the operating environment requires a thin-walled structure which is effective at elevated temperatures and/or in corrosive environments.
In such demanding conditions, two types of refractory materials have been used in the art, metals and ceramics. Each suffers from disadvantages. Although metals can be mechanically strong and relatively easy to shape into diverse structures of variable wall thicknesses, they typically are poor performers in environments including elevated temperatures or corrosive media (particularly acidic or oxidative environments). Although many ceramics can withstand demanding temperature and corrosive environments better than many metals, they are difficult to shape, suffer diminished strength compared to metals, and require thicker walls to compensate for their relative weakness compared to metals. In addition, chemical processes for making ceramics often are environmentally detrimental. Such processes can include toxic ingredients and waste. In addition, commonly used processes for making ceramic structures by sintering powders is a difficult manufacturing process which requires the use of very pure powders with grains of particular size to provide desirable densification of the material at high temperature and pressure. Often, the process results in cracks in the formed structure.
Metal oxides are useful ceramic materials. In particular, iron oxides in their high oxidation states, such as hematite (.alpha.-Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) are thermally stable refractory materials. For example, hematite is stable in air except at temperatures well in excess of 1400.degree. C., and the melting point of magnetite is 1594.degree. C. These iron oxides, in bulk, also are chemically stable in typical acidic, basic, and oxidative environments. Iron oxides such as magnetite and hematite have similar densities, exhibit similar coefficients of thermal expansion, and similar mechanical strength. The mechanical strength of these materials is superior to that of ceramic materials such as cordierite and other aluminosilicates. Hematite and magnetite differ substantially in their magnetic and electrical properties. Hematite is practically non-magnetic and non-conductive electrically. Magnetite, on the other hand, is ferromagnetic at temperatures below about 575.degree. C. and is highly conductive (about 10.sup.6 times greater than hematite). In addition, both hematite and magnetite are environmentally benign, which makes them particularly well-suited for applications where environmental or health concerns are important. In particular, these materials have no toxicological or other environmental limitations imposed by U.S. OSHA regulations.
Metal oxide structures have traditionally been manufactured by providing a mixture of metal oxide powders (as opposed to metal powders) and reinforcement components, forming the mass into a desired shape, and then sintering the powder into a final structure. However, these processes bear many disadvantages including some of those associated with processing other ceramic materials. In particular, they suffer from dimensional changes, generally require a binder or lubricant to pack the powder to be sintered, and suffer decreased porosity and increased shrinkage at higher sintering temperatures.
Use of metal powders has been reported for the manufacture of metal structures. However, formation of metal oxides by sintering metal powders has not been considered desirable. Indeed, formation of metal oxides during the sintering of metal powders is considered a detrimental effect which opposes the desired formation of metallic bonds. "Oxidation and especially the reaction of metals and of nonoxide ceramics with oxygen, has generally been considered an undesirable feature that needs to be prevented." Concise Encyclopedia of Advanced Ceramic Materials, R. J. Brock, ed., Max-Planck-Institut fur Metalforschung, Pergamon Press, pp. 124-25 (1991).
In the prior art, it has been unacceptable to use steel starting materials to manufacture uniform iron oxide structures, at least in part because oxidation has been incomplete in prior art processes. In addition, surface layers of iron oxides made according to prior art processes suffer from peeling off easily from the steel bulk.
Heat treatment of steels often has been referred to as annealing. Although annealing procedures are diverse, and can strongly modify or even improve some steel properties, the annealing occurs with only slight changes in the steel chemical composition. At elevated temperatures in the presence of oxygen, particularly in air, carbon and low alloy steels can be partially oxidized, but this penetrating oxidation has been universally considered detrimental. Such partially oxidized steel has been deemed useless and characterized as "burned" in the art, which has taught that "burned steel seldom can be salvaged and normally must be scrapped." "The Making, Shaping and Testing of Steel," U.S. Steel, 10th ed., Section 3, p. 730. "Annealing is . . . used to remove thin oxide films from powders that tarnished during prolonged storage or exposure to humidity." Metals Handbook, Vol. 7, p. 182, Powder Metallurgy, ASM (9th Ed. 1984).
One attempt to manufacture a metal oxide by oxidation of a parent metal is described in U.S. Pat. No. 4,713,360. The '360 patent describes a self-supporting ceramic body produced by oxidation of a molten parent metal to form a polycrystalline material consisting essentially of the oxidation reaction product of the parent metal with a vapor-phase oxidant and, optionally, one or more unoxidized constituents of the parent metal. The '360 patent describes that the parent metal and the oxidant apparently form a favorable polycrystalline oxidation reaction product having a surface free energy relationship with the molten parent metal such that within some portion of a temperature region in which the parent metal is molten, at least some of the grain intersections (i.e., grain boundaries or three-grain-intersections) of the polycrystalline oxidation reaction product are replaced by planar or linear channels of molten metal.
Structures formed according to the methods described in the '360 patent require formation of molten metal prior to oxidation of the metal. In addition, the materials formed according to such processes does not greatly improve strength as compared to the sintering processes known in the art. The metal structure originally present cannot be maintained since the metal must be melted in order to form the metal oxide. Thus, after the ceramic structure is formed, whose thickness is not specified, it is shaped to the final product.
Another attempt to manufacture a metal oxide by oxidation of a parent metal is described in U.S. Pat. No. 5,093,178. The '178 patent describes a flow divider which it states can be produced by shaping the flow divider from metallic aluminum through extrusion or winding, then converting it to hydrated aluminum oxide through anodic oxidation while it is slowly moving down into an electrolyte bath, and finally converting it to .alpha.-alumina through heat treatment. The '178 patent relates to an unwieldy electrochemical process which is expensive and requires strong acids which are corrosive and environmentally detrimental. The process requires slow movement of the structure into the electrolyte, apparently to provide a fresh surface for oxidation, and permits only partial oxidation. Moreover, the oxidation step of the process of the '178 patent produces a hydrated oxide which then must be treated further to produce a usable working body. In addition, the description of the '178 patent is limited to processing aluminum, and does not suggest that the process might be applicable to iron or other metals. See also, "Directed Metal Oxidation," in The Encyclopedia of Advanced Materials, vol. 1, pg. 641 (Bloor et al., eds., 1994).
Accordingly, there is a need for metal oxide structures which are of high strength, efficiently and inexpensively manufactured in environmentally benign processes, and capable of providing refractory characteristics such as are required in demanding temperature and chemical environments. There also is a need for metal oxide structures which are capable of operating in demanding environments, and having a variety of shapes and wall thicknesses.